A subset of human T cells express T cell receptors (TCR) composed of gamma and delta subunits, homologous to alpha and beta proteins but encoded by distinct gene segments. The rapid response of gamma/delta T cells to microbial epithelial invasion suggests they represent a first line of defense for innate immunity. Others have shown that gamma/delta T cells provide a link between innate and adaptive immunity, producing cytokines such as IFN-gamma important for inducing type 1 immunity. We have identified a more direct adaptive immune role for gamma/delta T cells. We found that BCG vaccination induces human gamma/delta memory T cells characterized by more rapid and potent secondary responses. Vaccination with canarypox vectors and vaccinia also induced human gamma/delta memory T cell responses. The major focus of this RO1 has been to determine whether these memory gamma/delta T cell responses provide important helper and/or effector functions involved in protective immunity, and we have produced evidence for both. This competing continuation application will further investigate the following 3 hypotheses: #1) Mycobacteria- and poxviral-specific gamma/delta T cells have unique antigen specificities and can be used as bioindicators to identify pathogen derived stimulatory antigens, #2) Human gamma/delta T cells specific for mycobacterial and poxviral antigens can provide helper functions for the development of optimal memory immune alpha/beta T cell responses, and #3) Human gamma/delta T cells specific for mycobacteria and poxviruses can provide direct effector functions capable of inhibiting the replication of these intracellular pathogens. Studies of smallpox-specific gamma/delta T cell immunity, detailed TCR spectratyping comparisons between gamma/delta T cells induced by these different pathogens/vectors, identification of the pathogenic-specific antigenic molecules inducing these responses, and detailed studies of the mechanisms involved in gamma/delta T cell helper and effector immune functions are major additions in this application cycle. In addition, Aims 2 &3 will specifically address in detail the antigen specificity of gamma/delta T cell responses. Another major addition is the inclusion of a subcontract to Dr. Dobos at Colorado State, who will pursue detailed biochemical studies designed to identify the specific components of mycobacteria and vaccinia that induce gamma/delta T cells relevant for vaccine development.